Estimate the concrete for a known number of fence posts, accounting for the space occupied by each submerged post. Enter the hole and post dimensions, then review both the combined material count and a separate per-hole bag estimate.
Enter the posts and concrete zone
The regular path starts with the number of post holes, hole shape and size, and the height of the concrete fill. Enter the portion of the post that is actually surrounded by concrete, not the post’s total above-ground length. Posts may be round or rectangular, and holes may be circular or rectangular. If the post begins above the concrete, only the immersed segment displaces concrete.
There are two different ways to describe a hole depth. The default asks directly for concrete fill height, so a gravel layer is already outside the entered concrete zone and is not deducted. An optional mode can instead take total hole depth and a bottom-layer height, then subtract once to find the concrete zone. These modes are mutually exclusive. Entering a fill height and also subtracting a base layer would remove the same depth twice.
Subtract the submerged post volume
For a round hole, calculate the cylinder volume using its radius and concrete height. For a rectangular hole, multiply its inside length, width, and fill height. Then subtract the volume of the post segment inside that same concrete zone: net fill = hole volume − submerged post volume. Multiply by the number of identical holes and apply the chosen material allowance to the combined net volume. Keep dimensions in a consistent unit before doing the subtraction.
A round hole must physically contain the post cross-section. A square post inside a circular hole is not valid merely because the square’s area is smaller than the circle’s area; its corner-to-corner diagonal must fit. The calculator should flag an impossible geometry rather than produce a negative or misleading net volume. It should also reject a submerged post length greater than the height of the concrete zone.
Worked example: ten holes with square posts
For ten circular holes 10 in in diameter with 30 in of concrete, each hole contains about 1.364 ft³. A square post measuring 3.5 × 3.5 in that extends through the full 30 in concrete zone occupies about 0.213 ft³. Subtracting gives approximately 1.151 ft³ per hole, or 11.509 ft³ for ten holes before an allowance. With 10% added to the combined project volume, the order quantity is about 12.660 ft³, or 0.46887 yd³.
If the selected product is QUIKRETE Fast-Setting Concrete Mix No. 1004 in a 50 lb bag, its data sheet lists an approximate yield of 0.375 ft³. Dividing 12.660 by 0.375 and rounding up gives 34 bags for a combined purchase. If you want to open complete bags separately at each hole, each hole’s adjusted quantity calls for four bags, or 40 bags total. These counts answer different purchase approaches and should be labeled separately.
Optional post-count helper for a straight run
If you know a straight fence segment’s length and a maximum spacing, the optional helper can estimate how many intervals are needed. A 20 ft line with maximum spacing of 8 ft needs three intervals because the length divided by spacing is rounded up; a line with three intervals has four points. This is only a count helper for one explicitly described straight segment. It does not add a guessed corner post, gate post, brace post, or terminal post.
If you enter a post count directly, the calculator should not add end posts again. For a fence with several segments, tell the tool which endpoints are shared before subtracting shared posts. Corners, gates, grade changes, and openings require a real layout. The count helper cannot infer them from total fence length. Once the count is set, the concrete volume calculation uses that count and the per-hole dimensions.
Choose a product and buy the right amount
The fence-post path can default to the named 50 lb Fast-Setting Concrete Mix No. 1004 product, whose listed approximate yield is 0.375 ft³. The 60 lb version is listed at about 0.45 ft³. Product and bag size should remain visible in the result because another fast-setting mix can have a different yield. Do not assume a package yield from the words fast setting or from weight alone; check the current product data and availability.
Before purchasing, compare combined-project rounding with the way you intend to work. Buying enough bags for all holes as one pooled quantity can use fewer bags than opening whole bags separately at every hole. The tool should expose both methods rather than hide that difference. It does not choose embedment, concrete depth, post dimensions, drainage, or fence construction details. Use the project plan and local conditions to determine those values.
Limits of this estimate
This calculator estimates fill volume for the dimensions you enter. It does not recommend hole diameter, burial depth, post spacing, footing depth, bracing, or a method for setting a fence. Soil, wind exposure, fence height, post material, gates, local frost conditions, and site drainage can affect those decisions. A suitable bag count does not show that the fence is stable or complies with local requirements.
Results also depend on the hole actually matching its simple cylinder or rectangular model. Irregular sides, sloughed soil, standing water, bell-shaped bases, and enlarged bottoms can change volume. Measure representative holes and account for special shapes separately. If a post touches or intersects the hole boundary, resolve the geometry rather than relying on a small difference between two calculated volumes. For one or a few isolated holes without a fence run, the post-hole calculator presents the same subtraction in a more general single-hole workflow.
Frequently asked questions
Does the tool subtract the post from each hole?
Yes. It subtracts only the post length entered as submerged in the concrete zone, then multiplies the net fill by the hole count.
Should I enter total hole depth or concrete depth?
The default path asks for concrete depth directly. Use the separate total-depth mode only when also entering a bottom layer to subtract once.
Why can a square post fail to fit in a round hole?
Its diagonal can exceed the hole diameter even when the square’s area is smaller than the circle’s area. The geometry must fit at every corner.
Why are there two bag counts?
One rounds after totaling all holes; the other rounds up each hole independently. The per-hole method may require more unopened bags.
What is the yield of a 50 lb fast-setting bag?
QUIKRETE Fast-Setting Concrete Mix No. 1004 lists approximately 0.375 ft³ per 50 lb bag. Confirm the exact product data before purchase.
Does the calculator tell me how deep to set fence posts?
No. It estimates material for a fill depth you enter and does not select embedment or evaluate fence stability.